The invention concerns the preparation of ethyl tertiobutyl ether (abbreviated to ETBE).
It is known that ethyl tertiobutyl ether, like methyl tertiobutyl ether (abbreviated to MTBE), may be used as an additive with a high octane number for lead-free petrols (opsolines) or petrols with a low lead content. The addition of ETBE to petrols may be envisaged, at concentrations, e.g.; up to about 15% by volume.
A method of preparing MTBE comprises carrying out a reaction in which methanol is added to isobutene, contained, e.g.) in a C.sub.4 cut from steam cracking, catalytic cracking or dehydrogenation of isobutane. After the reaction, the residual methanol is generally hydrocarbons. This makes it fairly easy to obtain MTBE of a purity suitable for addition to petrols.
ETBE can be prepared by a similar process, with the methanol replaced by ethanol. Such a process is described, e.g., in "1'ETBE, un avenir pour 1'ethanol (ETBE, a future for ethanol" by A. FORESTIERE, B. TORCK and G. PLUCHE, a paper read at the Conference on Biomass for Energy and Industry, Lisbon, 9-13 Oct. 1989 and in "MTBE/ETBE, an Incentive Flexibility for Refiners" by A. FORESTIERE et coll., a paper read at the Conference on Oxygenated Fuels in Europe, London, 22-23 May 1990.
In contrast with the MTBE process, however, when the C.sub.4 hydrocarbons have been removed in this process, nearly all the residual ethanol is found to be mixed with the ETBE produced. The existence of an ethanol-ETBE azeotrope containing 21% by weight of ethanol at atmospheric pressure and boiling at 66.6.degree. C. makes it difficult to separate ETBE of a purity sufficient to satisfy specifications on the ethanol content of petrols. Thus the ethanol content of ETBE must generally be from 5 to 10% by weight. ETBE will advantageously have to be purified to less than 2% by weight of ethanol in order to be conveyed to a refinery.
Thus there were great hopes of finding an economically attractive separating method, to enable ETBE to compete with MTBE as an additive to improve the octane number of lead-free petrols. This is what the invention proposes.